Dilip Srinivas Sundaram

Combustion of Nano Aluminum Particles (Review)

Nano aluminum particles have received considerable attention in the combustion community; their physicochemical properties are quite favorable as compared with those of their micron-sized counterparts. The present work provides a comprehensive review of recent advances in the field of combustion of nano aluminum particles. The effect of the Knudsen number on heat and mass transfer properties of particles is first examined. Deficiencies of the currently available continuum models for combustion of nano aluminum particles are highlighted. Key physicochemical processes of particle combustion are identified and their respective time scales are compared to determine the combustion mechanisms for different particle sizes and pressures. Experimental data from several sources are gathered to elucidate the effect of the particle size on the flame temperature of aluminum particles. The flame structure and the combustion modes of aluminum particles are examined for wide ranges of pressures, particle sizes, and oxidizers. Key mechanisms that dictate the combustion behaviors are discussed. Measured burning times of nano aluminum particles are surveyed. The effects of the pressure, temperature, particle size, and type and concentration of the oxidizer on the burning time are discussed. A new correlation for the burning time of nano aluminum particles is established. Major outstanding issues to be addressed in the future work are identified.

Combustion of Frozen Nanoaluminum and Water Mixtures

mixtureexhibitedalinearburningrateof4. 8c m∕satapressureof10.7MPaandapressureexponentof0.79.Three motors of internal diameters in the range of 1.91–7.62 cm were studied. Grain configuration, nozzle throat diameter, and igniter strength were varied. The propellants were successfully ignited and combusted in each laboratory-scale motor, generating thrust levels above 992 N in the 7.62-cm-diam motor with a center-perforated grain configuration (7.62 cm length) and an expansion ratio of 10. For the 7.62 cm motor, combustion efficiency was 69%, whereas the specific impulse efficiency was 64%.Increased combustionefficiency and improvedease of ignitionwere observedat higher chamber pressures (greater than 8 MPa).

Flame Propagation of Nanoaluminum-Water Mixtures

A theoretical investigation on the combustion behavior of nano-aluminum (nAl) and liquid water is conducted. Linear burning rates of nAl and liquid water as a function of pressure and equivalence ratio are reported. A 38-nm diameter aluminum particle with a 3.1-nm thick oxide layer has been chosen for the present study. The simulation is carried out for a pressure range of 0.1 - 3.65 MPa. The equivalence ratio considered in the present study varies from 0.5 to 1.25. The model predicts an increase in linear burning rate from 0.7 to 8.1 cm/s at a pressure of 3.65 MPa, as the equivalence ratio is increased from 0.5 to 1.25. The predicted burning rates are in good agreement with the experimentally measured values. The flame thickness predicted by the model is 108 microns.

Combustion of Aluminum, Aluminum Hydride, and Ice Mixtures

The combustion of nano-aluminum, alane, and ice mixtures is theoretically studied. A multi-zone theoretical framework is established by solving the energy equation in each zone and matching the temperature distribution and the heat flux at the interfacial boundaries. The effect of replacing a portion of nano-aluminum particles with micron-sized aluminum and alane particles is examined in the pressure range of 1-10 MPa and for an additive mass fraction of 25%. The addition of micron-sized alane particles results in lower flame temperatures due to the dehydrogenation reaction of alane particles prior to their ignition. The lower flame temperatures and the longer burning times of micron-sized alane particles is responsible for lower flame speeds of these mixtures. For bimodal aluminum-ice mixtures, the lower mass fraction of alumina causes an increase in the flame temperatures. However, the mixtures exhibit mildly lower flame speeds, in view of the longer burning times of micron-sized aluminum particles. The flame thickness of a bimodal mixture of aluminum particle mixtures is higher than that of mono-dispersed mixtures due to the prevalence of two distinct reaction zones. The model results are well supported by experimentally measured burning rates.

Heat transport in aqueous suspensions of alumina nanoparticles

Equilibrium molecular dynamics simulations are conducted to investigate heat transport in aqueous suspensions of alumina nanoparticles. The thermal conductivity of the suspension, calculated by the Green-Kubo relations, is studied for a wide range of volume fractions, particle sizes, and temperatures. The particle volume fraction is varied in the range of 1-9% and the particle size range of concern is 1-9 nm. The temperature varies between 300 and 370 K. The radial distribution function and the radial density profiles are utilized to estimate the thickness of the ordered base-fluid nanolayer adsorbed on the particle surface. Emphasis is placed on elucidating the relationship between the thermal conductivity enhancement and the nanolayer thickness. Results show that the effective thermal conductivity increases near-linearly as a function of volume fraction, whereas the slope of this function decreases with an increase in particle size. The nanolayer thickness is independent of the particle volume fraction. The effect of particle size on thermal conductivity is studied for a volume fraction of 5%, temperature of 300 K, and pressure of 1 atm. The nanolayer thickness remains almost constant with an increase in particle size. However, both effective thermal conductivity, and nanolayer thickness normalized with particle diameter decreases sharply with increasing particle size and attains an asymptotic value at particle diameter ~ 150 nm. The effect of temperature on the effective thermal conductivity is studied for a particle size of 3 nm and volume fraction of 5%. The thermal conductivity decreases steadily with increasing temperature, whereas the nanolayer thickness remains nearly constant. For particle sizes less than 10 nm, the enhancement in thermal conductivity is significantly greater than the predictions of existing theoretical models. A strong correlation between nanolayer properties and enhanced thermal conductivity of fully dispersed nanoparticle suspensions can be deduced from the results.

Thermo-Mechanical Behavior of Nickel-Coated Nano- Aluminum Particles

Thermo-mechanical behavior of nickel coated nano-aluminum particles, in the size range of 4-16 nm, is studied using molecular dynamics simulations. The analysis is carried out in isothermal-isobaric and isochoric-isoenergetic ensembles using an embedded atom method. Emphasis is laid on analyzing the melting of Al core, diffusion of Al and Ni atoms, and intermetallic reactions for different core sizes and shell thicknesses. The melting point of the Al core is found to exceed the heterogeneous melting point of pure nAl particle and approach the homogenous melting point of Al, irrespective of the shell thickness. The diffusion of Al atoms, after melting, is accompanied by self-sustaining inter-metallic reactions between Al and Ni atoms. The advent of these reactions is, to some extent, delayed for a thicker shell and expedited for a larger core. The amount of heat release due to the reactions increases as the Al or Ni atomic fraction increases to 0.5. Adiabatic reaction temperatures close to 2300 K are predicted for near-equiatomic particles with a 1 nm thick Ni shell. The simulation results indicate the possibility of ignition of these particles in an inert environment and also help in explaining their reduced ignition temperatures.